Most errors in GNSS signal reception are known to be caused by nearby objects. This is due to the fact that an antenna receives not only the line-of-sight signal broadcast by a satellite but also signals reflected from the nearby objects (the so-called multipath reception). These objects can be divided into two types. The first type of objects are those that are distant from the antenna by several wavelengths (for example, five wavelengths) and even farther. Such effects caused by reflections from these objects may be called “Far-Field Multipath”. The underlying surface, primarily, the ground, is the main object of the first type. Objects distant from the antenna by no more than a few wavelengths belong to the second type. The effects caused by them are called “Near-Field Multipath”.
Means used for fixing the antenna are primarily related to this group. Antenna fittings are normally under the antenna in its vicinity (at distance of 1-2 wavelengths). A tribrach is often used in satellite geodesic/surveying applications to fix the antenna onto a surveying tripod. Designs of tribrachs are quite diverse. When antenna position over the tribrach changes, it often results in undesirable variation of antenna performance. The antenna phase center is affected by the tribrach. To provide a high quality reception of GNSS signals, the antenna has to suppress multipath signals caused by reflections from far-field objects (first type) and near-field objects (second type). In particular, the antenna phase center should be independent of fitting elements (for example, tribraches) and antenna positions relative to the fixing elements.
Antennas used in satellite positioning tasks are mostly receiving ones. However, in some instances, antenna performance/characteristics need to be considered for a transmission mode. Note that the reciprocity principle governs the identity of antenna characteristics in reception and transmission modes.
If the antenna operates in a transmission mode, rejection of multipath reception means that a field radiated by an antenna has to be low in the lower hemisphere area. However in the upper hemisphere, this field needs to be high. The latter condition is essential for high quality signal reception, including reception from low elevation satellites. FIG. 1 shows a generalized division of antenna space into top and bottom hemispheres with a horizontal dotted line.
A directional diagram is a common characteristic of antenna directivity. But it characterized the field only in the far field area (i.e., area distant from the antenna by a few wavelengths). To eliminate effects of near field objects (tribrach), one needs to minimize the field in the near field area as well. This field has a more complicated pattern. Also, a decrease of the field in the far field zone does not always result in a decreasing field in the near field zone.
Special ground planes are used to reduce the field in the bottom hemisphere. A flat conducting ground plane is of low effectiveness. Impedance choke ring ground planes are more efficient and widespread. Such a ground plane effectively suppress both far field and near field in the bottom hemisphere, but simultaneously reduces the field in the top hemisphere in the horizon direction. It also has considerable dimensions and weight.